Titanium metals and alloys are produced commercially as ingots, and then processed to finish parts by various forgoing, rolling, extrusion and machining operations. Some titanium metal parts are formed by casting molten titanium metal or alloys. For many of the operations, the titanium metals or alloys are heated to allow forming the metals with less force and to allow greater deformation of the material without cracking. Each time titanium metal or alloy is heated in air, it reacts with the oxygen therein to form a titanium oxide coating which is then removed prior to further fabrication of the metal. The oxide coating is removed by sand blasting, bead blasting, or abrasive techniques followed by acid pickling the metal to chemically removed small amounts of the titanium metal from the surface. Many titanium metal parts, particularly castings, are chemically milled to final dimensional tolerances by pickling or chemical milling in pickle acid baths.
The pickling operation consists of placing the titanium metal or alloy in a bath of hydrofluoric acid and water to yield a titanium metal fluoride (TiF.sub.4 or TiF.sub.3). The chemical reactions which occur are depending on availability of HF (the second reaction (II) also depends upon nitric acid or other oxidant being present in the bath): EQU Ti+3HF-TiF.sub.3 +3/2H.sub.2 (I) EQU TiF.sub.3 +F-TiF.sub.4 (II)
A typical pickling acid bath contains 2 to 7 weight-percent hydrofluoric acid. The rate of pickling is enhanced by adding a strong mineral acid to the bath. Both sulfuric and nitric acids are commonly used, typically at a concentration of between 5 and 35 weight-percent. Nitric acid has the additional advantage that it inhibits the absorption of hydrogen into the titanium metal or alloy during the pickling operations and, therefore, is the most commonly used mineral acid additive. The mineral acid also aids in the dissolution of some of the alloying ingredients in the titanium metal or alloy and in oxidizing trivalent titanium to tetravalent titanium in the pickle acid bath. To increase the bath life of the pickle acid bath, additional hydrofluoric and mineral acid are typically added periodically. The hydrofluoric acid reacts with the titanium to form a titanium metal fluoride. When the pickle acid becomes saturated with the titanium fluoride at about 65-80 grams of titanium per liter, depending on the pickle acid temperature, oxidation state of the titanium, or acid concentration, an impurity concentration, the acid bath is "spent". At this point the pickling reaction is greatly retarded and hydrous titanium fluoride solids coat the titanium metal or alloy being pickled, making further pickling unsatisfactory.
The spent titanium pickle acid contains titanium fluoride, mineral acid (typically nitric acid), sometimes excess hydrofluoric acid, and various dissolved alloying ingredients (usually vanadium and aluminum from the commonly processed 90% Ti-6% V-4% Al alloy).
My U.S. Pat. No. 4,105,469 teaches a process for treating spent pickle acid from zirconium pickling such that the zirconium fluoride salts are recovered and the pickle acid is regenerated for additional pickling. Fennemann (U.S. Pat. No. 4,330,342) improved this process by showing NaOH and extra HF could be used to recycle the spent zirconium pickle acid. During our research into various spent pickle acid systems we have found that various metal salts such as NaHCO.sub.3, Na.sub.2 CO.sub.3, NaHF, and NaNO.sub.3 can be used replacing NaF and NaOH as taught in these patents together with added hydrofluoric acid. These raw materials (particularly Na.sub.2 CO.sub.3) further reduce the cost of the process and allow for tighter process control as taught by Fennemann.
Any attempt to recycle titanium pickle acid solutions is limited by the large amounts of alloying ingredients commonly in titanium alloys which rapidly build up in recycle systems to render the pickle acid unusable.
Commercial attempts to recycle titanium pickle acid mixtures using a process similar to that taught in U.S. Pat. No. 4,105,469, but substituting KF for NaF fail in pickle acid systems which pickle titanium alloys due to the rapid buildup of impurity components in the recycled pickle acid, and impurities in the product salt. Even when one attempts to recover fluoride salts without attempting to recycle the pickle acid, the salt produced has high levels of impurities and fine particle size unless processed according to the teachings herein.
There are instances, however, where chemically pure ("CP") titanium is pickled. In this case, where no alloying metals are present, it is practical to adjust the spent titanium pickle acid with HF or AHF.sub.2 (where A is an Alkali metal) and treat with alkali metal salts in accordance with the procedures described herein and recover a relatively pure alkali metal titanium fluoride, A.sub.2 TiF.sub.6. Under these circumstances, and as long as there are sufficient losses and/or intentional bleed off to prevent the buildup to the point of significant precipitation with the A.sub.2 TiF.sub.6 of impurities such as Ca, Mg, Fe, and Si, the processed acid can be fortified with fresh HF and HNO.sub.3 and recycled back to the pickle bath. In this process reagent and temperature control of the crystallization process is important to avoid precipitation of fluoride salts in the regenerated pickle bath.
At the present time most spent pickle acid liquors are neutralized with caustic soda or lime and discarded. When spent titanium pickle acid liquors are neutralized with caustic, large amounts of solids which contain sodium, titanium, fluorine, and hydroxide approximately consistent with the stoichiometry of Na.sub.2 Ti(OH).sub.2 F.sub.4, are formed together with complexed solids of the alloying ingredients. These solids are commonly disposed of in landfills, but are objectionable due to the solubility of the fluorine ion in these solids. The liquid from the caustic neutralization also contains considerable fluoride ion which is objectionable in many liquid disposal options. An obvious option would be to use lime to neutralize the pickle liquor and fix the fluoride ion in the generated solids. Unfortunately, liming the pickle liquor gives a larger volume of solids which are difficult to filter.